DFT-based evaluation of covalent organic frameworks for adsorption, optoelectronic, clean energy storage, and gas sensor applications

Context

Covalent Organic Frameworks (COFs), which are frameworks composed of light atoms held together by strong covalent bonds, are generating interest as potential materials for applications such as renewable energy and gas capture. We employed Density Functional Theory (DFT) calculations, as implemented in the VASP code, to look at both 2D and 3D COFs. We systematically analyzed various properties, including structural stability, phonon dispersion, electronic structures, density of states, adsorption behavior, and mechanical properties. To get better accuracy, we took into account van der Waals interactions and even used hybrid functionals. What we found was that 3D COFs generally exhibit greater mechanical strength and, in most cases, better gas adsorption, which seems to come from their interconnected pore structures. On the other hand, 2D COFs exhibit enhanced π-electron delocalization and direct band gaps of approximately 2.5 eV, which may be helpful in sensors and optoelectronics. Phonon analyses verified the dynamical stability of both structures. Ultimately, these results underscore the importance of dimensionality in tailoring COF properties for energy and electronic applications.

Method

First-principles simulations were performed using Density Functional Theory (DFT) within the Vienna Ab initio Simulation Package (VASP). To account for exchange–correlation effects, we employed the Generalised Gradient Approximation (GGA) in the Perdew–Burke–Ernzerhof (PBE) formulation, and we also utilised projector-augmented wave (PAW) pseudopotentials. Hybrid functional (HSE06) and DFT-D3 van der Waals corrections were introduced to improve the accuracy of our band gap prediction. The plane-wave cutoffs were set at 500 eV for the calculations, and Monkhorst–Pack k-point meshes were used with a 3 × 3 × 1 (2D) and 2 × 2 × 2 (3D) grid. Evaluated were structural optimisations, band structures, total and projected DOS, adsorption energies, and charge transfer (using Bader analysis). Assessment of bonding features utilised the Electron Localisation Function (ELF) and charge density difference (Δρ) visualisations. The Phonopy package was used to calculate Phonon dispersions and thus confirm the dynamic stability of the COFs.